An in vitro system in which spleen cells from unprimed rabbits of known allotypes are stimulated with a soluble T2 phage antigen preparation, has been found to be uniquely suitable for studies on allotype suppression. Preliminary studies have shown that spleen cells from allotype-suppressed rabbits continue to exhibit a deficiency in the formation of the suppressed type of immunoglobulin even when stimulated with antigen. However, in the presence of rabbit anti-allotype serum specific for the non-suppressed allotype, escape from allotype suppression occurs. Further studies using this system are proposed, with the aim of identifying and characterizing the suppressed cell and elucidating the mechanism responsible for release of suppression. Suggestive evidence already obtained for the role of suppressor cells in rabbit allotype suppression will be used as the basis for more extensive investigation into the properties and mode of action of such cells. Information gained in the proposed studies could have relevance to more general aspects of immunoglobulin regulation which may play a role in cancer immunotherapy, immunodeficiency diseases, and rejection phenomena.